Add GBB flags to enable dev mode by default

And enable dev_boot_usb by default.
And disable rollback checks.

The first flag is necessary for factory to build with keyboard
controlled dev mode.  The other flags are really handy for development
on systems where you've defeated firmware WP and are installing custom
firmware.

BUG=chromium-os:31844
TEST=make && make runtests

Signed-off-by: Randall Spangler <rspangler@chromium.org>

Change-Id: I9d837fee676cb0186ea98f13005ad60a9ab86393
Reviewed-on: https://gerrit.chromium.org/gerrit/25265
Tested-by: Randall Spangler <rspangler@chromium.org>
Reviewed-by: Bill Richardson <wfrichar@chromium.org>
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
Commit-Ready: Randall Spangler <rspangler@chromium.org>
6 files changed